Set sshd not to use weak encryption

Checking sshd immediately after installing CentOS Stream 8 shows that it is not recommended for high security systems The encryption method and hash function are also set to be available. So, make a note of the procedure when you disabled them.

Settings that may be necessary as a security measure

Do not use cbc (Cipher Block Chaining) for cipher. VU # 958563
Do not use sha1 for MACs. SHA-1 is used as a component of HMAC-SHA1. As of 2020, there seems to be no problem with the safety of HMAC-SHA1. That said, it tends to avoid using SHA-1 itself, so it is invalidated.

Outline of setting procedure

Unlike the time of CentOS7, the setting is not reflected just by describing it in sshd_config. The setting method seems to be roughly the following two

Use the mechanism ** crypto-policies ** to set the encryption method in a batch adopted from CentOS8 However, the prepared policy cannot be disabled as intended, so create a custom policy.
Make settings that are not affected by ** crypto-policies ** and set with sshd_config as before.

It seems easier to do it in 2, but ... For the time being, I will try both.

crypt-policies custom policy configuration example

** This setting affects many services. ** ** Check the target service with man update-crypto-policies. When I confirmed it on 2021/1/3, the following items were targeted. GnuTLS library,OpenSSL library,NSS library,OpenJDK,Libkrb5,BIND,OpenSSH,Libreswan,libssh

Copy the standard policy: I tried using it based on FUTURE for high security
Edit the copied policy: Delete the one with cbc
Apply the created policy
Restart the target service

Copy the underlying policy

[user01@centos8st ~]$ sudo cp /usr/share/crypto-policies/policies/FUTURE.pol ¥
/usr/share/crypto-policies/policies/MYPOLICY.pol
[sudo]password for user01:
[user01@centos8st ~]$

Edit the copied policy

Removed CBC related from cipher

[user01@centos8st ~]$ sudo vi /usr/share/crypto-policies/policies/MYPOLICY.pol
# A level that will provide security on a conservative level that is
# believed to withstand any near-term future attacks. And also provide
# some (not complete) preparation for post quantum encryption support
# in form of 256 bit symmetric encryption requirement.
# It provides at least an 128-bit security. This level may prevent
# communication with many used systems that provide weaker security levels
# (e.g., systems that use SHA-1 as signature algorithm).

# MACs: all HMAC with SHA256 or better + all modern MACs (Poly1305 etc)
# Curves: all prime >= 255 bits (including Bernstein curves)
# Signature algorithms: with SHA-256 hash or better (no DSA)
# TLS Ciphers: >= 256-bit key, >= 128-bit block, only Authenticated Encryption (AE) ciphers
# non-TLS Ciphers: same as TLS Ciphers with added non AE ciphers and Camellia
# key exchange: ECDHE, DHE (no DHE-DSS)
# DH params size: >= 3072
# RSA params size: >= 3072
# TLS protocols: TLS >= 1.2, DTLS >= 1.2

mac = AEAD HMAC-SHA2-256 UMAC-128 HMAC-SHA2-384 HMAC-SHA2-512

group = X25519 X448 SECP256R1 SECP384R1 SECP521R1 \
    FFDHE-3072 FFDHE-4096 FFDHE-6144 FFDHE-8192

hash = SHA2-256 SHA2-384 SHA2-512 SHA3-256 SHA3-384 SHA3-512

sign = ECDSA-SHA3-256 ECDSA-SHA2-256 \
       ECDSA-SHA3-384 ECDSA-SHA2-384 \
       ECDSA-SHA3-512 ECDSA-SHA2-512 \
       EDDSA-ED25519 EDDSA-ED448 \
       RSA-PSS-SHA2-256 RSA-PSS-SHA2-384 RSA-PSS-SHA2-512 \
       RSA-SHA3-256 RSA-SHA2-256 \
       RSA-SHA3-384 RSA-SHA2-384 \
       RSA-SHA3-512 RSA-SHA2-512

tls_cipher = AES-256-GCM AES-256-CCM CHACHA20-POLY1305

cipher = AES-256-GCM AES-256-CCM CHACHA20-POLY1305 CAMELLIA-256-GCM \
    AES-256-CTR

key_exchange = ECDHE DHE DHE-RSA PSK DHE-PSK ECDHE-PSK ECDHE-GSS DHE-GSS

protocol = TLS1.3 TLS1.2 DTLS1.2
ike_protocol = IKEv2

min_tls_version = TLS1.2
min_dtls_version = DTLS1.2

# Parameter sizes
min_dh_size = 3072
min_dsa_size = 3072
min_rsa_size = 3072

# GnuTLS only for now
sha1_in_certs = 0

arbitrary_dh_groups = 1
ssh_certs = 1
ssh_etm = 1
[user01@centos8st ~]$

Apply the created policy / restart the target service

[user01@centos8st ~]$ sudo update-crypto-policies --set MYPOLICY
Setting system policy to MYPOLICY
Note: System-wide crypto policies are applied on application start-up.
It is recommended to restart the system for the change of policies
to fully take place.
[user01@centos8st ~]$ sudo systemctl restart sshd

State of encryption method / hash algorithm before and after setting change

I checked it with nmap from the outside.

Before changing settings

user01@kali:~$ nmap -p 22 192.168.1.122 -Pn --script ssh2-enum-algos.nse
Host discovery disabled (-Pn). All addresses will be marked 'up' and scan times will be slower.
Starting Nmap 7.91 ( https://nmap.org ) at 2020-12-30 17:48 JST
Nmap scan report for 192.168.1.122
Host is up (0.00091s latency).

PORT   STATE SERVICE
22/tcp open  ssh
| ssh2-enum-algos:
|   kex_algorithms: (11)
|       curve25519-sha256
|       [email protected]
|       ecdh-sha2-nistp256
|       ecdh-sha2-nistp384
|       ecdh-sha2-nistp521
|       diffie-hellman-group-exchange-sha256
|       diffie-hellman-group14-sha256
|       diffie-hellman-group16-sha512
|       diffie-hellman-group18-sha512
|       diffie-hellman-group-exchange-sha1
|       diffie-hellman-group14-sha1
|   server_host_key_algorithms: (5)
|       rsa-sha2-512
|       rsa-sha2-256
|       ssh-rsa
|       ecdsa-sha2-nistp256
|       ssh-ed25519
|   encryption_algorithms: (7)
|       [email protected]
|       [email protected]
|       aes256-ctr
|       aes256-cbc
|       [email protected]
|       aes128-ctr
|       aes128-cbc
|   mac_algorithms: (8)
|       [email protected]
|       [email protected]
|       [email protected]
|       [email protected]
|       hmac-sha2-256
|       hmac-sha1
|       [email protected]
|       hmac-sha2-512
|   compression_algorithms: (2)
|       none
|_      [email protected]

Nmap done: 1 IP address (1 host up) scanned in 0.24 seconds
user01@kali:~$

After changing the settings

CBC/SHA1-related cryptography/hash algorithms have been eliminated.

user01@kali:~$ nmap -p 22 192.168.1.122 -Pn -sV --script ssh2-enum-algos.nse
Host discovery disabled (-Pn). All addresses will be marked 'up' and scan times will be slower.
Starting Nmap 7.91 ( https://nmap.org ) at 2021-01-03 15:44 JST
Nmap scan report for 192.168.1.122
Host is up (0.00063s latency).

PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.0 (protocol 2.0)
| ssh2-enum-algos:
|   kex_algorithms: (8)
|       curve25519-sha256
|       [email protected]
|       ecdh-sha2-nistp256
|       ecdh-sha2-nistp384
|       ecdh-sha2-nistp521
|       diffie-hellman-group-exchange-sha256
|       diffie-hellman-group16-sha512
|       diffie-hellman-group18-sha512
|   server_host_key_algorithms: (4)
|       rsa-sha2-512
|       rsa-sha2-256
|       ecdsa-sha2-nistp256
|       ssh-ed25519
|   encryption_algorithms: (3)
|       [email protected]
|       [email protected]
|       aes256-ctr
|   mac_algorithms: (6)
|       [email protected]
|       [email protected]
|       [email protected]
|       hmac-sha2-256
|       [email protected]
|       hmac-sha2-512
|   compression_algorithms: (2)
|       none
|_      [email protected]

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 0.35 seconds
user01@kali:~$

When setting with sshd_config as before

Easy to understand with the old-fashioned setting method.
Since the range of influence is only sshd, this is better if you are not responsible for the entire system.

** Setting flow **

Check the encryption / hash algorithm currently set in sshd
Invalidate the CRYPTO_POLICY variable
Settings in sshd_config
Restart the sshd service

Check the currently set encryption method / hash algorithm

If you are using crypto-policies, you can check it with the ps command because the encryption method / hash algorithm etc. are set as command line options when the daemon is started. Based on the value obtained here, set by excluding unnecessary encryption methods / hash algorithms.

[user01@centos8st ~]$ ps -ef|grep sshd
root        4679       1  0 15:23 ?        00:00:00 /usr/sbin/sshd -D [email protected],[email protected],aes256-ctr,aes256-cbc,[email protected],aes128-ctr,aes128-cbc [email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha1,[email protected],hmac-sha2-512 -oGSSAPIKexAlgorithms=gss-curve25519-sha256-,gss-nistp256-sha256-,gss-group14-sha256-,gss-group16-sha512-,gss-gex-sha1-,gss-group14-sha1- -oKexAlgorithms=curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1 -oHostKeyAlgorithms=ecdsa-sha2-nistp256,[email protected],ecdsa-sha2-nistp384,[email protected],ecdsa-sha2-nistp521,[email protected],ssh-ed25519,[email protected],rsa-sha2-256,[email protected],rsa-sha2-512,[email protected],ssh-rsa,[email protected] -oPubkeyAcceptedKeyTypes=ecdsa-sha2-nistp256,[email protected],ecdsa-sha2-nistp384,[email protected],ecdsa-sha2-nistp521,[email protected],ssh-ed25519,[email protected],rsa-sha2-256,[email protected],rsa-sha2-512,[email protected],ssh-rsa,[email protected] -oCASignatureAlgorithms=ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-256,rsa-sha2-512,ssh-rsa

[user01@centos8st ~]$

Invalidation of CRYPTO_POLICY variable

You can disable CRYPTO_POLICY that is set when the daemon starts by uncommenting it and declaring the variable in the empty state. In this example, the description on line 18 is so.

[user01@centos8st ~]$ sudo vi /etc/sysconfig/sshd

     15 # System-wide crypto policy:
     16 # To opt-out, uncomment the following line
     17 # CRYPTO_POLICY=
     18 CRYPTO_POLICY=
[user01@centos8st ~]$

Settings in sshd_config and restart sshd

The settings on lines 34 and 35 have been added based on the value obtained by the ps command with unnecessary items removed.

[user01@centos8st ~]$ sudo vi /etc/ssh/sshd_config

     29 # This system is following system-wide crypto policy. The changes to
     30 # crypto properties (Ciphers, MACs, ...) will not have any effect here.
     31 # They will be overridden by command-line options passed to the server
     32 # on command line.
     33 # Please, check manual pages for update-crypto-policies(8) and sshd_config(5).
     34 [email protected],[email protected],aes256-ctr,[email protected],aes128-ctr
     35 [email protected],[email protected],[email protected],hmac-sha2-256,[email protected],hmac-sha2-512

[user01@centos8st ~]$ sudo systemctl restart sshd

Cryptography / hash algorithm status after setting change

user01@kali:~$ nmap -p 22 192.168.1.122 -Pn --script ssh2-enum-algos.nse
Host discovery disabled (-Pn). All addresses will be marked 'up' and scan times will be slower.
Starting Nmap 7.91 ( https://nmap.org ) at 2020-12-30 23:12 JST
Nmap scan report for 192.168.1.122
Host is up (0.00051s latency).

PORT   STATE SERVICE
22/tcp open  ssh
| ssh2-enum-algos:
|   kex_algorithms: (10)
|       curve25519-sha256
|       [email protected]
|       ecdh-sha2-nistp256
|       ecdh-sha2-nistp384
|       ecdh-sha2-nistp521
|       diffie-hellman-group-exchange-sha256
|       diffie-hellman-group16-sha512
|       diffie-hellman-group18-sha512
|       diffie-hellman-group14-sha256
|       diffie-hellman-group14-sha1
|   server_host_key_algorithms: (5)
|       rsa-sha2-512
|       rsa-sha2-256
|       ssh-rsa
|       ecdsa-sha2-nistp256
|       ssh-ed25519
|   encryption_algorithms: (5)
|       [email protected]
|       [email protected]
|       aes256-ctr
|       [email protected]
|       aes128-ctr
|   mac_algorithms: (6)
|       [email protected]
|       [email protected]
|       [email protected]
|       hmac-sha2-256
|       [email protected]
|       hmac-sha2-512
|   compression_algorithms: (2)
|       none
|_      [email protected]

Nmap done: 1 IP address (1 host up) scanned in 0.21 seconds
user01@kali:~$

By the way

Since ssh connection as root is allowed, let's disable it. Set PermitRootLogin to no.

[user01@centos8st ~]$ sudo vi /etc/ssh/sshd_config

     42 # Authentication:
     43
     44 #LoginGraceTime 2m
     45 PermitRootLogin no
     46 #StrictModes yes
     47 #MaxAuthTries 6
     48 #MaxSessions 10

[user01@centos8st ~]$ sudo systemctl restart sshd
[user01@centos8st ~]$ ssh root@localhost
root@localhost's password:
Permission denied, please try again.
root@localhost's password:

reference: Akahat Engineer Blog How to customize crypto policies in RHEL 8.2

CentOS Stream8: Setting the encryption method used by sshd